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Advantages and disadvantages of gallium nitride

wallpapers News 2021-04-01
Advantages of gallium nitride
1. With a large bandgap (3.4eV) and high thermal conductivity (1.3W/cm-K), the working temperature is high, the breakdown voltage is high, and the radiation resistance is strong;
2. The bottom of the conduction band is at the Γ point, and the energy difference between it and the other energy valleys of the conduction band is large, it is not easy to produce inter-valley scattering so that a high strong field drift velocity can be obtained (the electron drift velocity is not easy to be saturated);
3. GaN is easy to form mixed crystals with AlN, InN, etc., and can be made into various heterostructures. The 2-DEG with mobility of 105cm2/Vs at low temperature has been obtained (because the 2-DEG has a higher surface density, it can effectively shield Factors such as optical phonon scattering, ionized impurity scattering, and piezoelectric scattering are included);
4. The lattice symmetry is relatively low (hexagonal wurtzite structure or tetragonal metastable zinc-blende structure), and has strong piezoelectricity (due to non-centrosymmetric) and ferroelectricity (spontaneously along the hexagonal c-axis) Polarization): Strong piezoelectric polarization (polarizing electric field up to 2MV/cm) and spontaneous polarization (polarizing electric field up to 3MV/cm) are generated near the heterojunction interface, which induces a very high density of interface charges, The energy band structure of the heterojunction is strongly modulated, and the two-dimensional space restriction of 2-DEG is strengthened, thereby increasing the areal density of 2-DEG (in AlGaN/GaN heterojunction, it can reach 1013/cm2, which is higher than that of AlGaAs /GaAs heterojunction is one order of magnitude higher), which is very meaningful for device operation.
 
Disadvantages and problems of gallium nitride
On the one hand, in theory, due to its energy band structure, the effective mass of the carriers is larger and the transport properties are poor, so the low electric field mobility is low and the high-frequency performance is poor.
On the other hand, the GaN single crystal grown by heteroepitaxial (using sapphire and SiC as the substrate) technology is still not satisfactory (this hinders the development of GaN devices), for example, the dislocation density has reached 108 ~1010/cm2 (Although the crystal structure of sapphire and SiC is similar to that of GaN, there are still relatively large lattice mismatches and thermal mismatches); the room temperature background carrier (electron) concentration of undoped GaN is as high as 1017cm-3 (It may be related to N vacancies, substitutional Si, substitutional O, etc.), and exhibits n-type conductivity; although it is easy to achieve n-type doping (Si doping can obtain electron concentration of 1015~1020/cm3, room temperature mobility> 300 cm2/Vs of n-type GaN), but the p-type doping level is too low (mainly doped with Mg), the resulting hole concentration is only 1017~1018/cm3, the mobility is <10cm2/Vs, and the doping efficiency is only 0.1%~ 1% (may be caused by the compensation of H and the higher self-ionization energy of Mg).
 
In short, from the overall point of view, the advantages of gallium nitride make up for its shortcomings, especially through the role of heterojunction, its effective transport performance is not inferior to GaAs, and the effect of making microwave power devices (microwave output power density) ) It is often far superior to all existing semiconductor materials.

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